The present invention is in the field of control of electrical machines and, more particularly, control of electrical machines employed for starting a turbine engine and for generating electrical power with the engine.
In a prior-art electrical system, a starter-generator may function as a machine to impart initial rotation of the engine. After the engine is running under its own power, the starter-generator may function as a generator to supply electrical power. Such a system may be employed in an aircraft.
Aircraft designs are continuously evolving. One concept of this design revolution has become known as “more electric aircraft” (MEA). In the context of MEA designs, starter-generator designs have evolved from simple brush-type direct current machines into newer designs of starter-generators which may be brushless synchronous machines.
As compared to brush-type DC machines, prior-art brushless synchronous machines may provide higher starting torque and long term reliability. But prior-art operation of a synchronous machine as a starter requires input of high-amperage variable frequency power. In a prior art synchronous machine, variable frequency power may be applied to a main-stator to produce a variable rotor speed. This variable frequency power must be provided with high-power converters, typically having a current capacity in excess of 200 amperes. Such high-power converters and their associated controls may add complexity, expense and weight to an aircraft.
A starter-generator on an aircraft may be driven by a turbine engine that may vary in speed during operation of the aircraft. When a prior-art synchronous machine is employed as an engine-driven generator, its output frequency may vary as a function of engine speed. Power conversion systems may be employed to convert variable frequency current into constant frequency current that may be used by aircraft electrical systems. Such prior-art conversion systems may employ high-power rectifiers and inverters which may add complexity, cost and weight to the aircraft.
As can be seen, there is a need to eliminate a requirement for high-power converters in a vehicle that employs a brushless synchronous starter-generator (i.e. electric) machine. Additionally, there is a need to provide such a starter-generator which may be driven with constant-frequency electrical power to produce starting torque over a variable speed range. Also there is a need to generate constant frequency power with such a machine while driving the machine at a variable speed.